As one of the vacuum-refining processes of molten steel, the RH-type vacuum-refining process is known, which comprises:
using a vacuum tank for refining molten steel, which tank includes a vacuum pump, a pair of vertical openings being provided on a bottom wall of said vacuum tank, a pair of legs having a respective inner bore for passing molten steel therethrough, each fitted into each of said vertical openings, for protecting said vertical openings from molten steel, and a pair of immersion tubes each of which is vertically connected to the lower end of each of said legs so as to project downwardly from the bottom wall of said vacuum tank;
immersing said pair of immersion tubes into molten steel received in a ladle arranged below said vacuum tank;
blowing an inert gas from one of said pair of immersion tubes while reducing the pressure in said vacuum tank by means of said vacuum pump to suck up molten steel received in said ladle, through the one of said immersion tubes and the one of said legs connected thereto, into said vacuum tank; and
returning molten steel sucked up into said vacuum tank, through the other one of said legs and the other one of said immersion tubes connected thereto, into said ladle to circulate molten steel received into said ladle between said ladle and said tank;
thereby degassing, in said vacuum tank, molten steel received in said ladle to refine said molten steel.
In the above-mentioned RH-type vacuum-refining process of molten steel, each of the pair of legs for protecting each of the pair of vertical openings on the bottom wall of the vacuum tank from molten steel comprises an annular brick wall formed by piling up through a joint material a plurality of magnesia-chrome fired radial bricks in a plurality of horizontal annular rows arranged one on the other so as to form an inner bore for passing molten steel therethrough. The leg is fitted through a joint material into each of the pair of vertical openings on the bottom wall of the vacuum tank for refining molten steel.
The inner bore of the leg is susceptible to serious erosion because molten steel including an inert gas vigorously flows therethrough. The leg is therefore formed with the use of magnesia-chrome fired radial bricks excellent in erosion resistance against molten steel.
FIG. 1 is a schematic longitudinal sectional view illustrating a conventional leg for a vacuum tank used in the RH-type vacuum-refining of molten steel; and
FIG. 2 is a schematic plan view illustrating the leg shown in FIG. 1. As shown in FIGS. 1 and 2, the conventional leg 1 comprises an annular brick wall formed by piling up zigzag, through a joint material of magnesia mortar, a plurality of magnesia-chrome fired radial bricks 3 in a plurality of horizontal annular rows arranged one on the other so as to form an inner bore 2 for passing molten steel therethrough. The lowermost row of the plurality of horizontal annular rows arranged one on the other which form the annular brick wall has an offset portion 6 for securing the leg 1 to a bottom wall 4 of the vacuum tank for refining molten steel by being hooked on a fitting 5 fixed to the bottom wall 4. The leg 1 is fitted through a joint material of magnesia mortar into each of a pair of vertical openings 7 of the bottom wall 4 with the offset portion 6 hooked on the fitting 5.
However, the above-mentioned conventional leg 1 has the following drawbacks. The magnesia-chrome fired radial bricks 3 which form the leg 1, while being excellent in erosion resistance against molten steel, have a high thermal expansion. As a result, each of the magnesia-chrome bricks 3 largely expands under the effect of the heat applied by molten steel passing through the inner bore 2 of the leg 1, tending to largely expand outwardly in the radial direction of the leg 1. The magnesia-chrome bricks 3 which form the uppermost row of the horizontal annular rows arranged one on the other, being in contact with bottom bricks 8 which form the upper portion of the bottom wall 4 of the vacuum tank, are inhibited from expanding. However, the magnesia-chrome bricks 3 which form rows other than the uppermost one, being in contact with a high-alumina castable layer 9 having a high thermal contraction which forms the middle and lower portions of the bottom wall 4, are not inhibited from expanding. As a result, horizontal and vertical joint materials between adjacent magnesia-chrome bricks 3 which form the leg 1 become loose, and erosion of the joint materials cannot be disregarded. In spite of the sufficient thickness of the magnesia-chrome bricks 3, therefore, the leg 1 becomes unusable after a relatively small number of vacuum-refining cycles of molten steel. In FIG. 1, bottom lining bricks 10 are provided under the bottom bricks 8.
Under such circumstances, there is a strong demand for development of a leg for a vacuum tank for refining molten steel, which, when vacuum-refining molten steel by the RH-type vacuum-refining process, is capable of withstanding the use for many cycles of vacuum-refining. A leg for a vacuum tank for refining molten steel provided with such a property has, however, not as yet been proposed.